With the sharp increase of communication traffic volume, licensed frequency spectrum of 3GPP is increasingly not enough to provide higher network capacity. In order to further improve the utilization of spectrum resources, 3GPP is discussing how to use an unlicensed frequency spectrum with the help of a licensed frequency spectrum, such as frequency bands of 2.4 GHz and 5 GHz. At present, the unlicensed frequency spectrums being used mainly include Wi-Fi, Bluetooth, radar, medical systems, and the like.
Under normal circumstances, the access technologies designed for licensed frequency bands, such as LTE (Long Term Evolution), are not suitable for use in unlicensed frequency bands, because the access technologies, such as LTE, have very high requirements of the spectral efficiency and user experience optimization. However, the carrier aggregation (CA) function provides a possibility for deployment of the LTE in an unlicensed frequency band. 3GPP proposes a concept of LAA (LTE Assisted Access) to use an unlicensed frequency spectrum with the help of an LTE licensed frequency spectrum. An unlicensed frequency spectrum may have two operation modes, one is supplemental downlink (SDL), that is, there are only downlink transmission sub-frames; and the other is a TDD (Time Division Duplexing) mode, including not only downlink sub-frames, but also uplink sub-frames. The case of SDL can only be used with the help of the carrier aggregation technology. However, the TDD mode can not only be used with the help of the carrier aggregation technology, but also be used with the help of the DC (Dual Connectivity), and can also be used independently.
Compared to a Wi-Fi system, an LTE system operating in an unlicensed frequency band is capable of providing a higher spectral efficiency and a greater coverage effect, whilst allowing seamless switching of data traffic between a licensed frequency band and an unlicensed frequency band based on a same core network. For users, this means a better broadband experience, a higher rate, a better stability and a mobile convenience.
The existing access technology used in an unlicensed frequency spectrum, such as Wi-Fi, has a weak anti-interference ability. To avoid interference, the Wi-Fi system designs a lot of interference avoidance regulations, such as CSMA/CD (Carrier Sense Multiple Access/Collision Detection). The basic principle of this method is that before an AP (access point) or a terminal of the Wi-Fi sends a signaling or data, it is necessary to first sensor whether other APs or other terminals around are sending/receiving signalings or data, and, if other APs or other terminals around are sending/receiving signalings or data, continue sensing, until no signaling or data transmission is sensed; or otherwise, generate a random number as a backoff time, within which, if no signaling or data transmission is detected, then on completion of the backoff time, the AP or the terminal may start to send a signaling or data. This process is shown in FIG. 1.
However, an LTE network ensures an interference level because of its very good orthogonality, so the uplink and downlink transmission between a base station and a user does not need to consider whether other base stations or other users around are transmitting data. If LTE is used in an unlicensed frequency band without considering whether other devices around are using the unlicensed band, then it will bring a great interference to a Wi-Fi device. As long as there is traffic, LTE transmits traffic without any sensing rule, so that the Wi-Fi device cannot transmit traffic when the LTE is transmitting traffic. Only until the LTE completes traffic transmission, is it possible to detect an idle state of a channel for data transmission.
It is thus clear that when an LTE network uses an unlicensed frequency band, one of the most important key points is to ensure that LAA is capable of coexisting with an existing access technology (such as Wi-Fi) on a fair and friendly basis. The conventional LTE system does not have a LBT (Listen Before Talk) mechanism to avoid collision.
Therefore, on the premise that normal operation of an LTE system in an unlicensed frequency band can be ensured, how to prevent the LTE system from generating significant interference with regard to other systems when operating in the unlicensed frequency band becomes a technical problem in urgent need of solution.